Titanium dioxide particles doped with rare earth element

ABSTRACT

Titanium dioxide particles is doped with a rare earth element. The doping ratio of the rare earth element is within a range more than 0 at % and not more than 5.0 at %, and the rare earth element is substituted for titanium atoms in the unit lattice of titanium dioxide so that the titanium dioxide particles emit light attributable to the rare earth element when the titanium dioxide particles are irradiated with light having the absorption wavelength of titanium dioxide and showing a peak at 360 nm.

CROSS-REFERENCE TO RELATED APPLICATION

This is a divisional application of Ser. No. 11/922,342 filed on Dec.17, 2007, which claims the priority of Japanese Patent Application No.2005-177077 filed on Jun. 17, 2007, the content of which isincorporated.

TECHNICAL FIELD

This invention relates to titanium dioxide particles doped with a rareearth element. More particularly, the present invention relates totitanium dioxide particles containing a rare earth element substitutingat the titanium sites of a titanium dioxide lattice.

BACKGROUND ART

Titanium dioxide is being widely utilized for pigments, cosmetics suchas UV creams, photocatalysts, solar cells and so on. It is also beingbroadly studied. Particularly, nano particles of titanium dioxide areattracting attention because of having various potential applications.Novel techniques relating to titanium dioxide include inserting europiumions (Eu³⁺) among nano sheets of titanium oxide to prepare a lightemitting material that emits red light attributable to Eu³⁺ (see, interalia, Non-Patent Document 1).

Another technique relating to titanium dioxide is a method ofmanufacturing ceramic powder directly from a mixture or the reactionproduct of a metal compound and an alkoxide by way of a thermal plasmaprocess (see, inter alia, Patent Document 1).

FIG. 17 schematically illustrates the light emitting mechanism of alight emitting material prepared by inserting europium ions among nanosheets of titanium oxide.

According to known technique of the above cited Non-Patent Document 1,nano sheets of titanium oxide is irradiated with light to exciteelectrons in a valence band (VB) to move to a conduction band (CD).Electrons that are excited to move to the conduction band are notrelaxed to move back directly to the valence band but trappedtemporarily in a defect level. Later, such electrons move to theexcitation level of Eu³⁺ without being recombined with holes in thevalence band. As a result, it becomes possible to take out fluorescenceattributable to Eu³⁺.

Such a move of energy from nano sheets of titanium oxide to Eu³⁺ takesplace effectively when the energy level of the defect level of the nanosheets of titanium oxide is slightly higher than that of Eu³⁺ in anexcited state. Since it is only necessary to excite nano sheets oftitanium oxide that is a host compound in order to take out fluorescenceattributable to rare earth ions, it is possible to use light of awavelength that is absorbed by the nano sheets of titanium oxide.

On the other hand, Patent Document 1 describes a method of manufacturingceramic powder, using (I) a metal compound other than alkoxide havingone or more than one metal elements, (II) an alkoxide having one or morethan one metal elements, a mixture of (I) and (I) and/or the reactionproduct of (I) and (II), by way of a thermal plasma process. With such athermal plasma process, it is possible to obtain ceramic powder withoutusing sintering and crushing steps.

With the technique described in Patent Document 1, it is possible tomanufacture highly crystalline ceramic powder by using a precursorobtained by mixing (I) and (II) or causing them to react with each otheras starting material.

REFERENCE DOCUMENTS

-   Patent Document 1: JP 05-9008-A-   Non-Patent Document 1: Xin et al., Appl. Phys. Lett. 2004, 85, 4187

DISCLOSURE OF THE INVENTION Problem to be Solved by the Invention

However, with the technique described in the above cited Non-PatentDocument 1, it is not possible to take out fluorescence attributable toterbium ions (Tb³⁺) because the energy level of the defect level of nanosheets of titanium oxide is lower than that of Tb³⁺ in an excited statewhen terbium ions (Tb³⁺) are inserted into nano sheets of titaniumoxide.

In other words, the above technique has a drawback that it can only takeout fluorescence attributable to limited rare earth ions that cansatisfy the above energy structure by inserting rare earth ions intonano sheets of titanium oxide.

While the technique described in the above-cited Patent Document 1 issuited for manufacturing multi-composite particles, it is not suited formanufacturing titanium dioxide particles doped with an impurity.

More specifically, an organic solvent or water is used when mixing (I)and (II) and the mixture solution is a slurry solution where (I) or (II)is partly precipitated. This means that, when a rare earth compound anda titanium alkoxide are selected for (I) and (II) respectively, theobtained titanium oxide particles can be those of a composite oxide oftitanium and a rare earth element or of titanium oxide and a rare earthoxide and not those where the rare earth element is substituted at thetitanium sites in the titanium oxide lattice.

Energy can hardly move from a host compound to a rare earth element inparticles of any of such composite oxides and composite matters andhence it is difficult to highly efficiently take out fluorescenceattributable to a rare earth element.

Therefore, it is the object of the present invention to provide titaniumdioxide particles, more particularly, titanium dioxide particles where arare earth element is substituted at the titanium sites from which it ispossible to highly efficiently take out fluorescence attributable to therare earth element and also a method of manufacturing such titaniumdioxide particles.

Means for Solving the Problem

According to the present invention, the above object is achieved byproviding a method of manufacturing titanium dioxide particles dopedwith a rare earth element, comprising: a step of preparing a liquidprecursor containing a titanium source and a rare earth metal source,the doping ratio of the rare earth element in the liquid precursor beingwithin a range not less than 0 at % and not more than 5.0 at %; a stepof generating thermal plasma; and a step of providing the liquidprecursor into the thermal plasma.

The doping ratio of the rare earth element in the liquid precursor maybe within a range not less than 0 at % and not more than 0.5 at %.

The titanium source may be a titanium alkoxide chelate complex and therare earth metal source may be a rare earth metal compound chelatecomplex.

The titanium alkoxide chelate complex may contain a titanium alkoxideselected from a group of titanium ethoxide, titanium propoxide andtitanium butoxide and an organic solvent selected from a group ofdiethanolamine, triethanolamine and acetylacetone and the rare earthmetal compound chelate complex may contain a rare earth metalnon-alkoxide selected from a group of rare earth nitrates, rare earthchlorides, rare earth sulfates and rare earth acetates, citric acid orethylenediaminetetraacetate and ammonia or a rare earth metal alkoxideselected from a group of rare earth metal ethoxides, rare earth metalpropoxides and rare earth metal butoxides and an organic solventselected from a group of diethanolamine, triethanolamine andacetylacetone.

The titanium source may be a titanium trichloride solution and the rareearth metal source may be a rare earth metal alkoxide.

The rare earth metal alkoxide may be selected from a group of rare earthmetal ethoxides, rare earth metal propoxides and rare earth metalbutoxides.

The rare earth element may be selected from a group of cerium,praseodymium, neodymium, promethium, samarium, europium, gadolinium,terbium, dysprosium, holmium, erbium, thulium and ytterbium.

When the rare earth metal compound chelate complex contains a rare earthmetal non-alkoxide selected from a group of rare earth nitrates, rareearth chlorides, rare earth sulfates and rare earth acetates, citricacid or ethylenediaminetetraacetate and ammonia, the preparation processfurther includes: a step of mixing the rare earth metal non-alkoxide andthe citric acid or ethylenediaminetetraacetate; a step next to themixing step of mixing the ammonia; and a step next to the mixing step ofmixing the titanium alkoxide chelate complex.

The step of mixing ammonia may be so conducted as to make the pH of therare earth metal compound chelate complex equal to 9.0.

The generating step may be conducted to generate oxygen-containingthermal plasma by means of a plasma generation system selected from agroup of an RF induction plasma system, a DC arc plasma system, a DC RFhybrid plasma system and a microwave induction plasma system.

The providing step may be conducted to atomize the liquid precursor.

In titanium dioxide particles doped with a rare earth element accordingto the present invention, the doping ratio of the rare earth element iswithin a range not less than 0 at % and not more than 5.0 at % and therare earth element is substituted for titanium atoms in the unit latticeof titanium dioxide so that the titanium dioxide particles emit lightattributable to the rare earth element when the titanium dioxideparticles are irradiated with light having the absorption wavelength oftitanium dioxide in order to achieve the object.

The doping ratio of the rare earth element is within a range not lessthan 0 at % and not more than 0.5 at %.

The particle diameter of the titanium dioxide particles may be within arange not less than 5 nm and not more than 100 nm.

The rare earth element may be selected from a group of cerium,praseodymium, neodymium, promethium, samarium, europium, gadolinium,terbium, dysprosium, holmium, erbium, thulium and ytterbium.

The titanium dioxide particles may contain anatase titanium dioxideparticles and rutile titanium dioxide particles.

The content ratio of the anatase titanium dioxide particles relative tothe titanium dioxide particles may be within a range not less than 0 wt% and less than 100 wt %.

The emission of light attributable to the rare earth element may beproduced by irradiating the titanium dioxide particles with lightexceeding the absorption wavelength of titanium dioxide and capable ofexciting the rare earth element.

Advantages of the Invention

In titanium dioxide particles doped with a rare earth element accordingto the present invention, the doping ratio of the rare earth element iswithin a range not less than 0 at % and not more than 5.0 at %,preferably within a range not less than 0 at % and not more than 0.5 at%. The rare earth element is reliably substituted for the titanium atomsin the unit lattice of titanium dioxide when the doping ratio is withinthe above-defined range.

As the titanium dioxide particles are irradiated with light having theabsorption wavelength of titanium dioxide, energy moves highlyefficiently from the titanium dioxide to the rare earth element becausethe rare earth element exists at the titanium sites in the unit latticeof titanium dioxide. Then, as a result, the titanium dioxide particlescan emit light attributable to the rare earth element.

A method of manufacturing titanium dioxide particles doped with a rareearth element according to the present invention comprises: a step ofpreparing a liquid precursor containing a titanium source and a rareearth metal source; a step of generating thermal plasma; and a step ofproviding the liquid precursor into the thermal plasma.

As liquid sources are used respectively for the titanium source and therare earth metal source, it is possible to mix titanium and the rareearth without precipitation. Additionally, since the doping ratio of therare earth element in the liquid precursor is within a range not lessthan 0 at % and not more than 5.0 at %, it is possible not only toefficiently control the doping ratio but also substituting the rareearth element at the titanium sites.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flowchart of the manufacturing method according to thepresent invention, showing the manufacturing steps thereof.

FIG. 2 is a schematic illustration of an apparatus for manufacturingtitanium dioxide particles according to the present invention.

FIG. 3 is a flowchart of Embodiment 1 of manufacturing process accordingto the present invention.

FIG. 4 is a flowchart of Embodiment 2 of manufacturing process accordingto the present invention.

FIG. 5 is a graph illustrating the X-ray diffraction patterns of Ex1-1,Ex1-2 and Ex1-4.

FIG. 6 is an electronmicrograph of Ex1-4.

FIG. 7 is graph illustrating the Er³⁺ concentration dependency of theanatase titanium dioxide content ratio and the crystal particlediameter.

FIG. 8 is a graph illustrating the cathode luminescence of Ex1-4 andthat of erbium oxide.

FIG. 9 is a graph illustrating the X-ray diffraction patterns of Ex2-5,Ex2-7, Ex2-9, Ex2-10 and non-doped titanium dioxide.

FIG. 10 is a graph illustrating the X-ray diffraction patterns of Ex2-5,Ex2-7, Ex2-9, Ex2-10 and non-doped titanium dioxide.

FIG. 11 is a graph illustrating the O₂ flow rate dependency of thecontent ratio and the crystal particle diameter of rutile titaniumdioxide for Ex2-5.

FIG. 12 is a graph illustrating the Raman spectrums of Ex2-1, Ex2-2,Ex2-4, Ex2-7, Ex2-9, Ex2-10 and Eu₂Ti₂O₇.

FIG. 13 is a graph illustrating the UV-visible diffusion/reflectionspectrums of Ex2-1, Ex2-5, Ex2-10, non-doped titanium dioxide andEu₂Ti₂O₇.

FIG. 14 is graph illustrating the excitation spectrum of Ex2-5 andphotoluminescence spectrums of doped and non-doped titanium dioxideexcited in various different conditions.

FIG. 15 is a graph illustrating the photoluminescence spectrums ofEx2-5, Ex2-10, Eu₂O₃ and Eu₂Ti₂O₇.

FIG. 16 is a graph illustrating the Eu³⁺ concentration dependency oflight emission intensity.

FIG. 17 is a schematic illustration of the light emitting mechanism of alight emitting material prepared by inserting europium ions among nanosheets of titanium oxide according to known technique.

EXPLANATION OF REFERENCE SYMBOLS

-   200: plasma reactor-   210: chamber-   220: power supply source-   230: atomizing probe-   240: sheath-   250: filter-   260: vacuum pump-   270: thermal plasma-   280: plasma torch

BEST MODE FOR CARRYING OUT THE INVENTION

Before describing embodiments of the present invention, the principleunderlying the present invention will be described below.

FIG. 1 is a flowchart of the manufacturing method according to thepresent invention, showing the manufacturing steps thereof.

FIG. 2 is a schematic illustration of an apparatus for manufacturingtitanium dioxide particles according to the present invention.

The steps of the manufacturing method will be described by referring tothe plasma reactor 200 in FIG. 2.

Step S110: A liquid precursor containing a titanium source and a rareearth metal source is prepared.

Both the titanium source and the rare earth metal source are solutions.The doping ratio of the rare earth element in the liquid precursor isregulated so as to be within a range not less than 0 at % and not morethan 5.0 at %. With the above-cited doping ratio, it is possible tosubstitute the rare earth element for all the titanium sites in thetitanium dioxide lattice and the crystal structure of titanium dioxidecan be maintained.

For example, a report on doping gadolinium oxide, which is similar totitanium dioxide, with a rare earth element (Eu) up to 14 at % and areport on doping yttrium oxide with a rare earth element (Tb) up to 6 at% are known. Preferably, the doping ratio of the rare earth element inthe liquid precursor is not less than 0 at % and not more than 0.5 at %.The process of substitution proceeds without problem without specifyingany particular manufacturing conditions when the content ratio of therare earth element is within the above range.

As will be described hereinafter, the intensity of emitted lightattributable to the rare earth element (rare earth ions) tends to fallwhen the rare earth element is doped beyond 5.0 at % so that it isdesirable that the content ratio of the rare earth element is not morethan 5.0 at %.

The expression “doping of a rare earth element” as used in thisspecification refers to that the rare earth element is substituted forpart of the titanium at the titanium sites of a titanium dioxide latticeand it should be noted that any composite particles of titanium dioxideand the rare earth element are not contained there.

Step S120: Thermal plasma 270 is generated in the chamber 210 of theplasma reactor 200.

The pressure in the chamber 210 is adjusted to a range between 10 and760 Torr by means of the vacuum pump 260. The power supply source 220generates plasma by means of a frequency of 0.2 MHz to 50 MHz and RFpower of 5 KW to 500 KW. Thermal plasma 270 is generated from gas thatcontains O₂ (oxygen gas) and may typically be Ar/O₂ thermal plasmagenerated from a mixture gas of O₂ and Ar supplied from the sheath 240.The power supply source 220 may be selected from a group of an REinduction plasma system, a DC arc plasma system, a DC RF hybrid plasmasystem and a microwave induction plasma system.

Step S130: The liquid precursor prepared in Step S110 is supplied to thethermal plasma 270. The liquid precursor may be supplied to the chamber210 by way of an atomizing probe 230. At this time, the liquid precursormay be supplied to the chamber 210 with Ar or O₂ carrier gas. As aresult, the liquid precursor turns into mist. Since such a misty liquidprecursor can be supplied continuously to the thermal plasma 270, it ispossible to produce uniform high quality powder. The front end of theatomizing probe 230 is arranged so as to be held in contact with thethermal plasma 270 generated by the power supply source 220. As a matterof course, the liquid precursor may be directly exposed to the thermalplasma 270.

The misty liquid precursor exposed to the thermal plasma 270 isdecomposed at high temperature. The decomposed liquid precursor givesrise to a chemical reaction when it is cooled. Then, as a result, stabletitanium dioxide particles are produced. Such a reaction process caneasily be conceivable to those skilled in the art by analogy. Theobtained titanium dioxide particles are collected by way of the chamber210 and the filter 250.

The starting material does not precipitate in the liquid precursor andis mixed well when liquid is used to supply the starting material andlimit the doping ratio of the rare earth element to the above-describedrange. Then, as a result, it is possible to obtain titanium dioxideparticles where the rare earth element is substituted at the titaniumsites. The particle diameter of the obtained titanium dioxide particlescan be within a range between 5 nm and 100 nm as a result of using athermal plasma process. Thus, it is possible to obtain titanium dioxideparticles having a desired particle diameter by controlling themanufacturing process.

Since the rare earth element is located at the titanium sites, therelationship between the energy level of titanium dioxide and that ofrare earth element ions, which is described earlier by referring to FIG.17, allows energy to move from titanium dioxide to the rare earthelement highly efficiently. This is because the physical distancebetween titanium dioxide and the rare earth element is small if comparedwith the distance between the titanium oxide nano sheets, which operateas light emitting material, and the rare earth element ions as describedin the Non-Patent Document 1 so that interactions can satisfactorilytake place.

Now, embodiments of the present invention will be described below byreferring to the accompanying drawings.

Embodiment 1

FIG. 3 is a flowchart of Embodiment 1 of manufacturing process accordingto the present invention.

Since the steps in FIG. 3 are same as those of FIG. 1 except Step S310,Step S120 and Step S130 will not be described below any further.

Step S310: A mixture containing a titanium alkoxide chelate complex anda rare earth metal compound chelate complex is prepared. The titaniumalkoxide chelate complex and the rare earth metal compound chelatecomplex respectively operate as a titanium source and a rare earth metalsource. Both such a titanium alkoxide chelate complex and such a rareearth metal compound chelate complex are liquid. The doping ratio of therare earth element in the liquid precursor is adjusted to be within arange not less than 0 at % and not more than 5.0 at %.

More particularly, Step S310 may be either Step S310A or Step S310B.

Step S310A: A liquid precursor containing a titanium alkoxide chelatecomplex, which by turn contains a titanium alkoxide that operates astitanium source and an organic solvent, and a rare earth metal compoundchelate complex, which by turn contains a rare earth metal non-alkoxideand citric acid or ethylenediaminetetraacetate (EDTA) and ammonia asrare earth metal source, is prepared.

The titanium alkoxide is selected from a group of titanium ethoxide,titanium propoxide and titanium butoxide. The organic solvent isselected from a group of diethanolamine, triethanolamine andacetylacetone. The above-described organic solvent can operate to stablyhold the titanium alkoxide in the solution without precipitation.

The rare earth metal non-alkoxide is selected from a group of rare earthnitrates, rare earth chlorides, rare earth metal sulfates and rare earthacetates. Citric acid or EDTA can operate to stably hold the rare earthmetal non-alkoxide in the solution without precipitation.

The rare earth element is selected from a group of cerium, praseodymium,neodymium, promethium, samarium, europium, gadolinium, terbium,dysprosium, holmium, erbium, thulium and ytterbium. Such a rare earthelement can be substituted for the titanium in the titanium dioxidelattice and emit light due to the atom size and the electronconfiguration thereof.

Ammonia operates to prevent citric acid or EDTA from hydrolyzing thetitanium alkoxide by turning the rare earth metal compound chelatecomplex that contains citric acid or EDTA, which is acidic, alkaline.Preferably, ammonia is mixed so as to make the pH of the rare earthmetal compound chelate complex equal to 9.0.

More specifically, firstly the rare earth metal non-alkoxide and citricacid or EDTA, which is acidic, are mixed with each other. As a result,the rare earth metal non-alkoxide is prevented from precipitating. Then,ammonia is mixed with the mixture solution to produce a rare earth metalcompound chelate complex. As a result, the pH of the rare earth metalcompound chelate complex is adjusted and the complex becomes alkaline.Thereafter, the rare earth metal compound chelate complex whose pH hasbeen adjusted is mixed with a titanium alkoxide chelate complex thatcontains titanium alkoxide and an organic solvent to prepare a liquidprecursor. As the mixings are conducted in the above-described order,the rare earth metal non-alkoxide and the titanium alkoxide areprevented from precipitating and additionally the titanium alkoxide isprevented from hydrolyzing.

The liquid precursor prepared in this way does not give rise to anyprecipitation because of the effect of the organic solvent and citricacid or EDTA and the titanium alkoxide chelate complex and the rareearth metal compound chelate complex can be mixed well in it. It shouldbe noted here that, as a result of intensive research efforts, theinventors of the present invention found a combination of an organicsolvent and citric acid or EDTA that operates to stably hold both thetitanium alkoxide and the rare earth metal non-alkoxide and mixes themwell.

Step S310B: A liquid precursor containing a titanium alkoxide chelatecomplex, which by turn contains a titanium alkoxide that operates astitanium source and an organic solvent, and a rare earth metal compoundchelate complex, which by turn contains a rare earth metal alkoxide thatoperates as rare earth metal source and an organic solvent, is prepared.

In this case, again, as in Step S310A, the titanium alkoxide is selectedfrom a group of titanium ethoxide, titanium propoxide and titaniumbutoxide. The organic solvent is selected from a group ofdiethanolamine, triethanolamine and acetylacetone.

The rare earth metal alkoxide is selected from a group of rare earthmetal ethoxides, rare earth metal propoxides and rare earth metalbutoxides. The organic solvent is selected from a group ofdiethanolamine, triethanolamine and acetylacetone as in the case of thetitanium alkoxide chelate complex. In this case again, the organicsolvent selected from the above group can operate to stably hold therare earth metal alkoxide in the solution without precipitation.

The rare earth element is selected from a group of cerium, praseodymium,neodymium, promethium, samarium, europium, gadolinium, terbium,dysprosium, holmium, erbium, thulium and ytterbium.

The titanium alkoxide chelate complex and the rare earth metal compoundchelate complex can be mixed with each other well without precipitatingdue to the effect of the organic solvent in the liquid precursor that isprepared in this way. It should be noted here that, as a result ofintensive research efforts, the inventors of the present invention founda combination of organic solvents that operates to stably hold both thetitanium alkoxide and the rare earth metal alkoxide and mixes them well.It is not necessary to adjust the pH in Step S310B.

The following steps may be same as Step S120 and Step S130 describedabove by referring to FIG. 1.

When the doping ratio of the rare earth element in the liquid precursoris adjusted within a range not less than 0 at % and not more than 0.5 at%, it is possible to obtain titanium dioxide particles doped with a rareearth element with ease because the manufacturing process does not needto be subjected to a number of restrictions including conditions forgenerating thermal plasma 270.

Embodiment 2

FIG. 4 is a flowchart of Embodiment 2 of manufacturing process accordingto the present invention.

Since the steps in FIG. 4 are same as those of FIG. 1 except Step S410,Step S120 and Step S130 will not be described below any further.

Step S410: A mixture containing a titanium trichloride solution and arare earth metal alkoxide is prepared. The titanium trichloride solutionand the rare earth metal alkoxide respectively operate as a titaniumsource and a rare earth metal source. Both such a titanium trichloridesolution and such a rare earth metal alkoxide are liquid. The dopingratio of the rare earth element in the liquid precursor is adjusted tobe within a range not less than 0 at % and not more than 5.0 at %.

The rare earth metal alkoxide is selected from a group of rare earthmetal ethoxides, rare earth metal propoxides and rare earth metalbutoxides. The rare earth element is selected from a group of cerium,praseodymium, neodymium, promethium, samarium, europium, gadolinium,terbium, dysprosium, holmium, erbium, thulium and ytterbium.

The titanium trichloride solution is more stable and more easilyhandleable if compared with the titanium alkoxide of Embodiment 1.Therefore, Embodiment 2 provides an advantage that the precursorsolution thereof can be prepared easier than Embodiment 1.

The following steps may be same as Step S120 and Step S130 describedabove by referring to FIG. 1.

As described above for Embodiment 1, when the doping ratio of the rareearth element in the liquid precursor is adjusted within a range notless than 0 at % and not more than 0.5 at % in Step S410, it is possibleto obtain titanium dioxide particles doped with a rare earth elementwith ease because the manufacturing process does not need to besubjected to a number of restrictions including conditions forgenerating thermal plasma 270.

Now, the present invention will be described further by way of examples,although it should be noted that the present invention is by no meanslimited to the examples.

Example 1

Specimens of titanium dioxide particles (hereinafter referred to asEr—TiO₂) doped with erbium to different doping ratios were manufacturedfrom respective liquid precursors, each containing a titanium alkoxidechelate complex, which by turn contains titanium butoxide as titaniumalkoxide and diethanolamine as organic solvent for stabilizing titaniumbutoxide, and a rare earth metal compound chelate complex, which by turncontains erbium nitrate as rare earth metal non-alkoxide and citricacid.

Referring to FIG. 2, the manufacturing system included astainless-steel-made reactor 210, an RF power supply system 220, astainless-steel-made filter 250 and a plasma torch (Model PL-50:tradename, available from TEKNA Plasma system Inc. Ltd., Canada) 280.

Each of the liquid precursors contained a titanium alkoxide chelatecomplex by turn containing 0.1 mol of titanium butoxide and 0.4 mol ofdiethanolamine, a rare earth metal non-alkoxide chelate complex by turncontaining erbium nitrate to a predetermined ratio (erbirum dopingratios of the specimens: 0.25 at %, 0.5 at %, 1.0 at % and 3.0 at %) andcitric acid and 20 ml of distilled water. Citric acid and erbium nitratewere mixed to show a mol ratio of 1:1.

Each of the liquid precursors obtained in this way was then supplied tothe center of thermal plasma 270 (FIG. 2) with Ar carrier gas (5 L/min)by way of an atomizing probe 230 (FIG. 2). The supply rate of the liquidprecursor was 4.5 g/min. The thermal plasma 270 was generated by amixture gas of Ar and O₂. The flow rate of the mixture gas was 90 L/min.The output of the RF power supply system 220 for generating thermalplasma was 25 kW and the pressure of the stainless-steel-made reactor210 was 500 torr. The above manufacturing conditions were listed inTable 1 below.

TABLE 1 Parameter numerical value sheath gas and flow rate Ar + O₂, 90L/min atomized gas and flow rate Ar, 5 L/min precursor supply rate 4.5g/min induction power 25 kW chamber pressure 500 torr

The produced titanium dioxide particles were collected from the lateralwall of the stainless-steel-made reactor 210 and the filter 250 asspecimens. The specimens of collected titanium dioxide particles arereferred to respectively as 0.25 at % Er—TiO₂ (Ex1-1), 0.5 at % Er—TiO₂(Ex1-2), 1.0 at % Er—TiO₂ (Ex1-3) and 3.0 at % Er—TiO₂ (Ex1-4) toindicate the erbium doping ratios.

The structures of Ex1-1 through Ex1-4 were analyzed by means of an X-raydiffractometer (Model Philips PW1800: tradename, available from PhilipsResearch Laboratories, Holland). The operating conditions of the X-raydiffractometer included an acceleration voltage using Cu—Kα rays of 40kV, an electric current of 50 mA and a scanning rate of 0.15°/2θ·min.For the purpose of simplicity, the results obtained for Ex1-1, Ex1-2 andEx1-4 are shown in FIG. 5 and will be described in detail below.

The surface of Ex1-4 was observed through a scanning electron microscope(FE-SEM, Model S-500: tradename, available from Hitachi, Japan).

The crystal sizes (particle diameters) of the specimens were determinedby means of the Scherrer's formula, using the diffraction peaks of the(101) face that indicates the anatase type and the (110) face thatindicates the rutile type obtained from FIG. 5. Additionally, thecontent ratio of the anatase titanium dioxide in the titanium dioxide ofeach of the specimens Ex1-1 through Ex1-4 was determined by means of theSpurr and Myers's formula. The results are shown in FIGS. 6 and 7 andwill be described below in detail.

The cathode luminescence of Ex1-4 was measured by means of a scanningelectron microscope (FE-SEM, Model S-4000SE: tradename, available fromHitachi, Japan) that was mounted by a cathode luminescence system.

The cathode luminescence measuring conditions included an acceleratingvoltage of 20 kV and a wavelength range between 1,100 nm and 1,600 nm.For the purpose of comparison, the results obtained for erbium oxide arealso shown in FIG. 8 and will be described below in detail.

FIG. 5 is a graph illustrating the X-ray diffraction patterns of Ex1-1,Ex1-2 and Ex1-4.

From the X-ray diffraction patterns, a peak of Er₂Ti₂O₇ of pyrochlorephase was detected from each of Ex1-3 (not shown in FIG. 3) and Ex1-4,which were doped with Er beyond 0.5 at %, from the X-ray diffractionpatterns.

While it is theoretically possible to dope up to 5.0 at %, it may besafe to assume that rare earth ions were deposited when the contentratio of the rare earth element exceeded 0.5 at % because themanufacturing process was not optimized. However, it was found that itis possible to obtain titanium dioxide particles doped with a rare earthelement with ease without providing a special manufacturing process whenthe content ratio is not less than 0 at % and not more than 0.5 at %.

Additionally, a peak of the (101) face that indicates anatase titaniumdioxide and a peak of the (110) face that indicates rutile titaniumdioxide were detected in each of the X-ray diffraction patterns of Ex1-1through Ex1-4. From the above, it was found that the obtained titaniumdioxide particles of each of the specimens were a mixture of anatasetitanium dioxide and rutile titanium dioxide.

Still additionally, it was also found the ratio of the peak intensity ofthe (101) face to that of the (110) face changes as a function of thedoping ratio of Er ions. This will be described in greater detailhereinafter by referring to FIG. 7.

FIG. 6 is an electronmicrograph of Ex1-4.

From FIG. 6, it was found that the obtained particles had particlediameters between 5 nm and 100 nm. While some of the particles wereagglomerated to show a diameter greater than 100 nm, it was found thatthe obtained particles were dispersed homogenously. As a result of anICP emission analysis on Ex1-4, it was found that the Er content ratiowas 5.71±0.02 wt %. This value approximately agreed with the fed-incontent ratio of 3 at % at the time of preparing the precursor (or 5.85wt %).

From the above finding, it was found that the doping ratio at thefeed-in time is maintained by a manufacturing method according to thepresent invention.

FIG. 7 is graph illustrating the Er³⁺ concentration dependency of theanatase titanium dioxide content ratio and the crystal particlediameter. From FIG. 7, it was found that the content ratio of anatasetitanium dioxide decreases as the Er³⁺ concentration increases. In otherwords, the content ratio of rutile titanium dioxide increases as theEr³⁺ concentration increases. This is caused by the generation of oxygenholes. As a trivalent Er³⁺ enters the position of a tetravalent Ti⁴⁺,oxygen holes can be generated according to formula (1) below.

formula (1):

Er₂O₃+3/2Ti_(Ti) ^(x)→2Er′_(Ti)+V₀{umlaut over ( )}+3/2TiO₂  (1),

where Ti_(Ti) ^(x) is Ti located at a Ti site and Er′_(Ti) is Er locatedat a Ti site, whereas V₀{umlaut over ( )} represents an oxygen hole. Itis known that rutile titanium dioxide has a capacity for generatingoxygen holes greater than anatase titanium dioxide. In other words,rutile titanium dioxide is generated with priority as Er³⁺ is dissolvedin titanium dioxide to produce oxygen holes.

More specifically, both the basic structure of rutile titanium dioxideand that of anatase titanium dioxide are a series of oxygen octahedrons,in each of which a titanium atom is surrounded by six oxygen atoms. Thenumber of linked edge sharing oxygen octahedrons is 2 in rutile titaniumdioxide, whereas that of linked edge sharing oxygen octahedrons is 4 inanatase titanium dioxide.

As oxygen atoms of oxygen octahedrons are drawn out to produce oxygenholes in anatase titanium dioxide where the number of linked edgesharing oxygen octahedrons is greater than in rutile titanium dioxide,the repulsive energy between the positive ions (Ti⁴⁺ in this case)located at the centers of two oxygen octahedrons becomes greater than inrutile titanium dioxide. Then, as a result, the crystal structure ofanatase titanium dioxide becomes instable so that rutile titaniumdioxide is generated with priority as a result.

While it was found that, while Er³⁺ ions are deposited as pyrochlorephase when the Er³⁺ concentration exceeds 0.5 at % from FIG. 5, anatasetitanium dioxide decreases (and hence rutile titanium dioxide increases)regardless that the Er³⁺ concentration exceeds 0.5 at %.

This suggests that Er³⁺ ions are dissolved in rutile titanium dioxide atthe time when cores are generated for titanium dioxide nano particles sothat it is possible to substitute Er³⁺ more for titanium dioxideparticles beyond 0.5 at % without generating a pyrochlore phase byoptimizing the manufacturing process. More specifically, it is possibleto suppress the generation of a pyrochlore phase by improving thecooling rate of a plasma downstream section. Then, as a result, it ispossible to obtain titanium dioxide particles doped with Er³⁺ maximallyto 5 at %.

FIG. 7 also suggests that the ratio of anatase titanium dioxide torutile titanium dioxide can be controlled by controlling themanufacturing conditions.

Anatase titanium dioxide can be utilized as photocatalyst. On the otherhand, rutile titanium dioxide has a refractive index and shows doublerefraction that are greater than anatase titanium dioxide so that it isexpected to be used as host materials and find optical applications inthe field of photonic crystals. Thus, it is possible to manufacturetitanium dioxide particles having a phase composition doped with a rareearth element according to the user application with ease.

The particle diameter of the rutile titanium dioxide in the obtainedtitanium dioxide particles was not less than 60 nm and it was found thatthe particle diameter grows up to 90 nm as the Er³⁺ concentrationincreases. The particle diameter of the anatase titanium dioxide in theobtained titanium dioxide particles was constantly about 35 nmregardless of the Er³⁺ concentration.

Now, the obtained results are summarily shown in Table 2 below.

TABLE 2 Doping anatase TiO₂ anatase TiO₂ rutile TiO₂ Specimen Dopingratio particle content:rutile avg particle avg particle No. element (at%) deposition TiO₂ content diameter (nm) diameter (nm) Ex 1-1 Er 0.25 no71:29 36 57 Ex 1-2 Er 0.5 no 67:33 35 63 Ex 1-3 Er 1.0 yes 64:36 35 71Ex 1-4 Er 3.0 yes 56:44 36 88

FIG. 8 is a graph illustrating the cathode luminescence of Ex1-4 andthat of erbium oxide. It was found that the pattern of Ex1-4 differsfrom that of erbium oxide. It was Confirmed that the pattern of Ex1-4agree with that of currently known Er-doped titanium dioxide. Morespecifically, the strongest peak (half width 9 nm) of Ex1-4 was found atabout 1,530 nm, which differs from the strongest peak (half width 22 nm)of erbium oxide that is located at about 1,534 nm.

From the above, the emission of light observed at about 1,530 nm was notattributable to free erbium oxide locally existing in titanium dioxideparticles but attributable to Er³⁺ substituted at the titanium sites oftitanium dioxide. In other words, the substitution was successful.

Example 2

Specimens of titanium dioxide particles (hereinafter referred to asEu—TiO₂) doped with europium to different doping ratios weremanufactured from respective liquid precursors, each containing atitanium alkoxide chelate complex, which by turn contains titaniumtetra-n-butoxide (TTBO) as titanium alkoxide and diethanolamine asorganic solvent for stabilizing TTBO, and a rare earth metal compoundchelate complex, which by turn contains europium nitrate as rare earthmetal non-alkoxide, citric acid and ammonia.

Each of the liquid precursors contained a titanium alkoxide chelatecomplex by turn containing 0.1 mol of TTBO and 0.4 mol ofdiethanolamine, a rare earth metal non-alkoxide chelate complex by turncontaining europium nitrate to predetermined ratio (europium dopingratios of the specimens: 0.05 at %, 0.1 at %, 0.2 at %, 0.3 at %, 0.5 at%, 0.75 at % 1.0 at %, 2.0 at %, 3.0 at % and 5.0 at %), citric acid andammonia and 20 ml of distilled water. Citric acid and europium nitratewere mixed to show a mol ratio of 1:1.

Since citric acid can hydrolyze TTBO in the presence of diethanolamine,the pH of the rare earth metal compound chelate complex was adjusted toabout 9.0 for each of the specimens by adding ammonia after mixingeuropium nitrate and citric acid. A 25% ammonia solution (3 mL) was usedfor the adjustment of pH. Thereafter, europium nitrate was mixed withthe rare earth metal compound chelate complex of each of the specimenswhose pH had been adjusted to obtain the corresponding liquid precursor.

Table 3 below shows the manufacturing apparatus and the manufacturingconditions of this example, which will not be explained further becausethey are same as those of Example 1 except the O₂ flow rate of theprecursor solution doped with europium to the ratio of 0.5 at % waschanged from 10 to 90 L/min (more specifically, 10, 30, 40, 50, 70 and90 L/min).

TABLE 3 parameter numerical value central gas and flow rate Ar, 30 L/minsheath gas and flow rate Ar + O₂, 90 L/min (O₂: 10, 30, 40, 50, 70, 90L/min) atomized gas and flow rate Ar, 5 L/min precursor supply rate 4.5g/min induction power 25 kW chamber pressure 500 torr

The produced titanium dioxide particles were collected from the lateralwall of the stainless-steel-made reactor 210 (FIG. 2) and the filter 250(FIG. 2) as specimens.

The specimens of collected titanium dioxide particles are referred torespectively as 0.05 at % Eu—TiO₂ (Ex2-1), 0.1 at % Eu—TiO₂ (Ex2-2), 0.2at % Eu—TiO₂ (Ex2-3), 0.3 at % Eu—TiO₂ (Ex2-4), 0.5 at % Eu—TiO₂(Ex2-5), 0.75 at % Eu—TiO₂ (Ex2-6), 1.0 at % Eu—TiO₂ (Ex2-7), 2.0 at %Eu—TiO₂ (Ex2-8), 3.0 at % Eu—TiO₂ (Ex2-9) and 5.0 at % Eu—TiO₂ (Ex2-10)to indicate the europium doping ratios.

The structures of Ex2-1 through Ex2-10 were analyzed by means of aninstrument similar to the one used in Example 1 under similar operatingconditions. For the purpose of simplicity, the results obtained forEx2-5, Ex2-7, Ex2-9 and Ex2-10 and those obtained for non-doped titaniumdioxide for purpose of comparison are shown in FIG. 9 and will bedescribed in detail below.

The crystal sizes (particle diameters) of the specimens were determinedby means of the Scherrer's formula, using the diffraction peaks of the(101) face that indicates the anatase type and the (110) face thatindicates the rutile type obtained from FIG. 9.

Additionally, the content ratios of the anatase titanium dioxide andrutile titanium dioxide in the obtained titanium dioxide particles ofthe specimens were determined by means of the Spurr and Myers's formulaas in Example 1. The results are shown in FIG. 10 and will be describedbelow in detail. The structure of the specimens of Ex2-5 obtained forthe different O₂ flow rates were also analyzed as in FIG. 9. The crystalsizes (particle diameters) and the content ratios of anatase titaniumdioxide and rutile titanium dioxide of the specimens were determined byusing the diffraction peaks. The obtained results are shown in FIG. 11and will be described in detail below. The specimens Ex2-1 throughEx2-10 are subjected to a Raman spectroscopy by means of a Ramanspectrometer (Model NR-1800: tradename, available from JASCO, Japan).

As for the conditions of the observation, the specimens were analyzedwith a resolution of 1 cm⁻¹, using Ar⁺ laser (wavelength: 514.5 nm;power; 50 mW) as the excitation light. The obtained results are shownfor Ex2-1, Ex2-2, Ex2-4, Ex2-7, Ex2-9 and Ex2-10 in FIG. 12 along withthe results obtained for Eu₂Ti₂O₇ of pyrochlore phase for the purpose ofcomparison and will be described in detail below.

The UV-visible diffusion/reflection spectrums of Ex2-1 through Ex2-10were observed by means of a visible-UV absorption spectrometer (JascoV-570: tradename, available from JASCO, Japan).

The observation wavelength was within a range between 200 nm and 800 nm.The results obtained for Ex2-1, Ex2-5 and Ex2-10 and also those obtainedfor non-doped titanium dioxide and Eu₂Ti₂O₇ of pyrochlore phase for thepurpose of comparison are shown in FIG. 13 and will be described indetail below.

The excitation spectrum and the emission spectrum of the specimens ofEx2-5 were observed by means of a fluorescence spectrophotometer(F-4500: tradename, available from Hitachi, Japan). The obtained resultsare shown in FIG. 14 and will be described in detail below.

The photoluminescence spectrum was observed for Ex2-1 through Ex2-10 bymeans of a photoluminescence spectrometer (Renishaw plc, UK). Thephotoluminescence was observed for Ex2-1 through Ex2-10 by means of aHe—Cd laser (wavelength 325 nm) for excitation light.

The results obtained for Ex2-5 and Ex2-10, and also those obtained fornon-doped titanium dioxide and Eu₂Ti₂O₇ of pyrochlore phase, which wereobserved for the purpose of comparison are shown in FIGS. 15 and 16 andwill be described in detail below.

FIG. 9 is a graph illustrating the X-ray diffraction patterns of Ex2-5,Ex2-7, Ex2-9, Ex2-10 and non-doped titanium dioxide.

As in Example 1, the peak (222) of Eu₂Ti₂O₇ of pyrochlore phase wasdetected from the X-ray diffraction patterns for Ex2-7, Ex2-9 and Ex2-10that were doped with Eu beyond 0.5 at %. However, it is possible to dopeup to 5.0 at % by optimizing the manufacturing process.

Additionally, as in Example 1, a peak of the (101) face that indicatesanatase titanium dioxide and a peak of the (110) face that indicatesrutile titanium dioxide were detected in each of the X-ray diffractionpatterns. From the above, it was found that the obtained titaniumdioxide particles of each of the specimens were a mixture of anatasetitanium dioxide and rutile titanium dioxide.

Still additionally, it was also found the ratio of the peak intensity ofthe (101) face to that of the (110) face changes as a function of thedoping ratio of Eu ions. This will be described in greater detailhereinafter by referring to FIG. 10.

FIG. 10 is a graph illustrating the Eu³⁺ concentration dependency of therutile titanium dioxide content ratio and the crystal particle diameter.From FIG. 10, it was found that the content ratio of rutile titaniumdioxide increases as the Eu³⁺ concentration increases.

More specifically, the content ratio of rutile titanium dioxide was 22wt % in non-doped titanium dioxide and increased to 52 wt % when dopedwith Eu³⁺ to a concentration of 5 at %. This means that it is possibleto control the content ratio of anatase titanium dioxide to rutiletitanium dioxide by controlling the manufacturing condition.

The fact that the content ratio of rutile titanium dioxide increaseswithout saturation as the Eu³⁺ concentration increases as in Example 1suggests that Eu ions can be substituted by optimizing the manufacturingprocess.

The particle diameter of the rutile titanium dioxide in the obtainedtitanium dioxide particles was not less than 60 nm and it was found thatthe particle diameter grows up to 90 nm as the Eu³⁺ concentrationincreases. The particle diameter of the anatase titanium dioxide in thetitanium dioxide particles was constantly between 30 nm and 35 nmregardless of the Eu³⁺ concentration.

FIG. 11 is a graph illustrating the O₂ flow rate dependency of thecontent ratio and the crystal particle diameter of rutile titaniumdioxide for Ex2-5. It was found that none of the content ratio of rutiletitanium dioxide, the particle diameter of rutile titanium dioxide andthe particle diameter of anatase titanium dioxide were dependent on theO₂ flow rate during the manufacturing process (that is the rate at whichO₂ is introduced into the sheath 240 (FIG. 2)). It was made clear fromFIGS. 10 and 11 that the phase composition and the crystal size(particle diameter) of the obtained titanium dioxide were dependent onthe doping ratio of the rare earth element.

Table 4 below summarily shows the above results.

TABLE 4 Doping O₂ anatase TiO₂ anatase TiO₂ rutile TiO₂ Specimen Dopingratio flow rate particle content:rutile avg particle avg particle No.element (at %) (L/min) deposition TiO₂ content diameter (nm) diameter(nm) Ex 2-1 Eu 0.05 40 no 77:23 33 68 Ex 2-2 Eu 0.1 40 no 74:26 35 65 Ex2-3 Eu 0.2 40 no 72.5:27.5 30 67 Ex 2-4 Eu 0.3 40 no 71:29 32 69 Ex 2-5Eu 0.5 10 to 90 no 69:31 35 68 Ex 2-6 Eu 0.75 40 yes 66:34 32 70 Ex 2-7Eu 1.0 40 yes 62.5:37.5 35 75.5 Ex 2-8 Eu 2.0 40 yes 56:44 31 78 Ex 2-9Eu 3.0 40 yes 52.5:47.5 35 80 Ex 2-10 Eu 5.0 40 yes 47.5:52.5 33 83

FIG. 12 is a graph illustrating the Raman spectrums of Ex2-1, Ex2-2,Ex2-4, Ex2-7, Ex2-9, Ex2-10 and Eu₂Ti₂O₇.

Eu₂Ti₂O₇ showed a diffused weak Raman scattering within a range between100 cm⁻¹ and 800 cm⁻¹. On the other hand, all the specimens of titaniumdioxide doped with Eu³⁺ (Ex2-1 through Ex2-10) showed a characteristicpeak.

More specifically, the scatterings at 146 cm⁻¹ (Eg mode), 200 cm⁻¹ (Egmode), 401 cm⁻¹ (Blg mode), 519 cm⁻¹ (Blg mode) and 641 cm⁻¹ (Eg mode)agreed with those of anatase titanium dioxide, where the scatterings at449 cm⁻¹ (Eg mode) and 614 cm⁻¹ (Alg mode) agreed with those of rutiletitanium dioxide.

The scattering at 143 cm⁻¹ (Blg mode) of rutile titanium dioxide did notshow any overlapping with the scattering at 146 cm⁻¹ (Eg mode) ofanatase titanium dioxide.

The peak intensity of the Eg mode (449 cm⁻¹) and that of the Alg mode(614 cm⁻¹) of rutile titanium dioxide increased as the Eu³⁺concentration increased. This fact agrees with that the content ratio ofrutile titanium dioxide increases as the Eu³⁺ concentration increases aspointed out above by referring to FIG. 10. Note that no peak of Eu₂Ti₂O₇was observed for Ex2-10. It may be safe to presume that this is becausethe pyrochlore phase contained in Ex2-10 was very small.

FIG. 13 is a graph illustrating, the UV-visible diffusion/reflectionspectrums of Ex2-1, Ex2-5, Ex2-10, non-doped titanium dioxide andEu₂Ti₂O₇.

All the spectrums had an absorption band at about 405 nm. It wasconfirmed that the absorption band corresponds to a band gap of about3.06 eV, which is same as the band gap of titanium dioxide.

On the other hand, Eu₂Ti₂O₇ showed additional absorptions at 395 nm, 416nm, 467 nm and 538 nm. These correspond to the inner shell transition4f→4f of Eu³⁺.

Meanwhile, none of Ex2-1, Ex2-5 and Ex2-10 showed peaks that correspondto the inner shell transition 4f→4f of Eu³⁺. This is because none of thespecimens contain Eu³⁺ to such an extent that allows the peaks of Eu³⁺to be detected and the transition process that causes the absorption ofEu³⁺ 395 nm that shows the highest intensity was quenched by the largeabsorption (about 405 nm) of titanium dioxide.

FIG. 14 is graph illustrating the excitation spectrum observed bymonitoring the 617 nm emission of Ex2-5 and non-doped titanium dioxideexcited in various different conditions.

Referring to FIG. 14, the spectrum (a) is the excitation spectrumobserved when Ex2-5 was excited by light having a wavelength of 617 nm.Note that the wavelength of 617 nm corresponds to radiation of Eu³⁺ dueto the ⁵D₀→⁷F₂ transition.

The spectrum (a) showed peaks at 360 nm, 416 nm, 467 nm and 538 nm. Thepeaks of 416 nm, 467 nm and 538 nm out of the four peaks agreed with thepeaks of the pyroclore phase of the UV-visible absorption spectrumdescribed above by referring to FIG. 13.

In other words, the three peaks correspond respectively to the⁷F_(0,1)→⁵D₃, ⁷F_(0,1)→⁵D₂ and ⁷F_(0,1)→⁵D₁ transitions of Eu³⁺. Thepeak of 360 nm in the spectrum (a) agreed with the absorption band ofnon-doped titanium dioxide described above by referring to FIG. 13.

From these, it was shown that Eu³⁺ was efficiently excited by way of thetitanium dioxide host lattice. Note that, although the right-hand skirtof the peak of 360 nm partly overlaps the peak of 416 nm, it does notcompletely overlap the latter peak.

The spectrum (b) is the photoluminescence spectrum that is observed whennon-doped titanium dioxide is excited with light of wavelength nothigher than 360 nm (wavelength equal to 360 nm here). The spectrum (b)did not show any characteristic peak.

The spectrum (d) is the photoluminescence spectrum that is observed whenEx2-5 is excited with light of wavelength not higher than 360 nm(wavelength equal to 360 nm here). Unlike the spectrum (b), it showedpeaks attributable to Eu³⁺ within a wavelength range between 590 nm and720 nm.

More particularly, the peaks of the spectrum (d) agree with the⁵D₀→⁷F_(j) (j=1 through 4) transitions of electrons of excited Eu³⁺.

These peaks can be visually identified as pure red. They are very sharpand show a high intensity. A light emitting material that shows such aclear red can be, for instance, effectively applied to white LEDs.

The spectrum (c) is the photoluminescence spectrum that is observed whenEx2-5 is excited with light having a wavelength greater than theabsorption edge of non-doped titanium dioxide (>450 nm) (wavelengthequal to 457 nm here).

Again, the spectrum (c) showed peaks attributable to Eu³⁺ within awavelength range between 590 nm and 720 nm.

Thus, it is possible to take out emitted light attributable to a rareearth element by irradiating titanium dioxide particles with light of awavelength that can excite rare earth elements (wavelength equal to 457nm here).

However, the peak intensities of the spectrum (c) are smaller that thoseof the spectrum (d) and the peaks were broader than those of thespectrum (d). From this fact, it was found that titanium dioxideparticles that are doped with a rare earth element according to thepresent invention can easily give rise to a move of energy from the hosttitanium dioxide to rare earth ions.

From above, it was also found that europium-doped titanium dioxideaccording to the present invention can easily excite Eu³⁺ by means oflight having the absorption wavelength of titanium dioxide to make itpossible to take out emitted light attributable to Eu³⁺.

FIG. 15 is a graph illustrating the photoluminescence spectrums ofEx2-5, Ex2-10, Eu₂O₃ and Eu₂Ti₂O₇. Both Ex2-5 and Ex2-10 showed peaksthat are different from the peaks of Eu₂O₃ and Eu₂Ti₂O₇. These peaks arethose of emitted red light that can be observed with naked eyes.

This fact suggests that the local environment of Eu³⁺ in Eu2-5 and inEu2-10 differs from that of Eu³⁺ in Eu₂O₃ and Eu₂Ti₂O₇. In other words,the radiation from Ex2-5 and Ex2-10 are not attributable to Eu₂O₃ andEu₂Ti₂O₇ but to Eu³⁺ substituted for titanium at part of the titaniumsites in the titanium dioxide lattice.

A peak similar to a peak attributable to Eu₂Ti₂O₇ was observed at 612 nmin the spectrum of Eu2-10. As described above by referring to FIG. 9,this is attributable to the deposition of a pyroclore phase due toexcessive Eu³⁺.

The spectrum of Eu2-5 showed the peaks of ⁵D₀→⁷F_(j) (j=1 through 4). Itis known that ⁵D₀→⁷F₁ rays (599 nm, three Stark splits) are producedfrom a magnetic dipole transition.

On the other hand, it is known that ⁵D₀→⁷F₂ rays (617 nm, five Starkdissociations) are produced from an electric dipole transition. Amagnetic dipole transition can take place while an electric dipoletransition is suppressed. An electric dipole transition can take placeonly when Eu³⁺ is not at the center of inversion but occupies latticepositions and is influenced by local symmetry.

As seen from the spectrum of Ex2-5 in FIG. 15, the peak intensity of theelectric dipole transition of Eu-doped titanium dioxide that is observedaccording to the present invention is greater than that of the magneticdipole transition. This fact suggests that Eu³⁺ is located at titaniumsites that are not at the center of inversion in the TiO₂ host latticein Eu-doped titanium dioxide according to the present invention.

It is well known that the relative intensity ratio of the ⁵D₀→⁷F₁transition to the ⁵D₀→⁷F₂ transition is highly sensitive to the localsymmetry of Eu³⁺. In Example 2, the intensity ratio of the ⁵D₀→⁷F₂transition (617 nm) to the ⁵D₀→⁷F₁ transition (599 nm) is constant andequal to about 9.7 in each of Ex2-1 through Ex2-10.

This fact suggests that the local environment of Eu³⁺ in each of thespecimens is such that it is located at the lattice position (ortitanium sites) in the TiO₂ host lattice.

FIG. 16 is a graph illustrating the Eu³⁺ concentration dependency oflight emission intensity.

The light emission intensity due to the ⁵D₀→⁷F₂ transition (617 nm) ofeach of Ex2-1 through Ex2-10 was normalized by the light emissionintensity of Ex2-5 on the basis of FIG. 15 and plotted in FIG. 16.

The luminance increased as the Eu³⁺ concentration increased up to 0.5 at% and then remained at a constant level beyond 0.5 at %. This is becauseEu³⁺ that contributes to emission light (or Eu³⁺ located at titaniumsites in the TiO₂ host lattice) is deposited as pyrochlore phase whenthe Eu³⁺ concentration exceeds 0.5 at % as described above by referringto FIG. 9.

Since the luminance increases in proportion to the concentration of Eu³⁺substituted at titanium sites, it is possible to obtain a light emittingmaterial showing a more desirable luminance level if it is possible todope Eu³⁺ ions up to 5.0 at % by optimizing the manufacturing process.

As described above in detail by way of examples and drawings, thepresent invention succeeded in producing titanium dioxide in which arare earth element is substituted at titanium sites in the titaniumdioxide crystal lattice thereof to a predetermined level and hencetitanium dioxide doped with a rare earth element. Thus, as a result, thepresent invention succeeded in producing a fluorescence emittingsubstance from which fluorescence can be highly efficiently taken out bymeans of a rare earth element. Namely, the applicability of the presentinvention is very high in many industrial fields as pointed out below.

INDUSTRIAL APPLICABILITY

Titanium dioxide particles doped with a rare earth element is describedabove in detail. It has been found that a rare earth element can besubstituted with ease at titanium sites in a TiO₂ host lattice withoutrequiring any complex manufacturing process within a concentration rangebetween 0 and 0.5 at %. It is also possible to substitute up to 5.0 at %by optimizing the manufacturing process.

Additionally, it is possible to take out light emitted from titaniumdioxide particles doped with a rare earth element that is attributableto rare earth ions simply by irradiating them with light having theabsorption wavelength of titanium dioxide. Thus, it is possible toutilize titanium dioxide particles doped with a rare earth element aslight emitting material by exploiting this light emittingcharacteristic. Light emitting materials in the form of nano particlescan find applications in the field of light emitting devices that can beused in white LEDs, plasma displays, optical amplifiers, micro lasers,television screens, lighting equipment and so on.

1. Titanium dioxide particles doped with a rare earth element, wherein adoping ratio of said rare earth element is within a range more than 0 at% and not more than 5.0 at %; and said rare earth element is substitutedfor titanium atoms in a unit lattice of titanium dioxide so that saidtitanium dioxide particles emit light attributable to said rare earthelement when said titanium dioxide particles are irradiated with lighthaving an absorption wavelength of titanium dioxide and showing a peakat 360 nm.
 2. The titanium dioxide particles according to claim 1,wherein the doping ratio of said rare earth element is within a rangemore than 0 at % and not more than 0.5 at %.
 3. The titanium dioxideparticles according to claim 1, wherein the particle diameter of saidtitanium dioxide particles is within a range not less than 5 nm and notmore than 100 nm.
 4. The titanium dioxide particles according to claim1, wherein said rare earth element is selected from the group consistingof cerium, praseodymium, neodymium, promethium, samarium, europium,gadolinium, terbium, dysprosium, holmium, erbium, thulium and ytterbium.5. The titanium dioxide particles according to claim 1, wherein saidtitanium dioxide particles contain anatase titanium dioxide particlesand rutile titanium dioxide particles.
 6. The titanium dioxide particlesaccording to claim 1, wherein the content ratio of the anatase titaniumdioxide particles relative to said titanium dioxide particles is withina range more than 0 wt % and less than 100 wt %.
 7. The titanium dioxideparticles according to claim 1, wherein the emission of lightattributable to said rare earth element is produced by irradiating saidtitanium dioxide particles with light exceeding the absorptionwavelength of titanium dioxide and capable of exciting said rare earthelement.